Triboelectrostatic charging

ABSTRACT

Surfaces are electrostatically charged using a triboelectrostatic effect enabling conventional corona discharge to be eliminated and making a grounded support unnecessary. The surface to be charged is rubbed against an electrode of one charge sign while it is passed through an electrical field between that electrode and a second electrode of opposite charge sign, both of which are positioned on the same side of the surface to be charged. At least one of the electrodes is surfaced with a low conductivity friction shield which is preferably made of chrysotile asbestos. Only one surface of a conventional photoconductive sheet is contacted, and small batteries are ample to power the electrodes which are supplied with high voltage at low amperage.

United States Patent Wagner [54] TRIBOELECTROSTATIC CHARGING [75] Inventor: Edward W. Wagner, Elmhurst, Ill.

[73] Assignee: DeSota, Inc., Des Plaines, Ill.

[22] Filed: June 5, 1972 [21] Appl. No.: 259,581

[52] US. Cl 317/262 A [51] Int. Cl G03g 15/02 [58] Field 0t Search 317/262 A, 4

[56] References Cited UNITED STATES PATENTS 2,774,921 12/1956 Walkup 317/262 A 3,376,208 4/l968 Wood 317/262 A Primary Examiner-L. T. Nix Attorney, Agent, or FirmDressler, Goldsmith,

Clement & Gordon, Ltd.

[ 1 Sept. 3, 1974 l 5 7 l ABSTRACT Surfaces are electrostatically charged using a triboelectrostatic effect enabling conventional corona discharge to be eliminated and making a grounded support unnecessary. The surface to be charged is rubbed against an electrode of one charge sign while it is passed through an electrical field between that electrode and a second electrode of opposite charge sign, both of which are positioned on the same side of the surface to be charged. At least one of the electrodes is surfaced with a low conductivity friction shield which is preferably made of chrysotile asbestos. Only one surface of a conventional photoconductive sheet is contacted, and small batteries are ample to power the electrodes which are supplied with high voltage at low amperage,

9 Claims, 4 Drawing Figures SINGLE PLANE RIC/4L F/ELO TRIBOELECTROSTATIC CHARGING The present invention relates to the electrostatic charging of surfaces utilizing a triboelectric effect which eliminates the need for either corona discharge or a grounded support for the surface being charged. As a result, the apparatus required for electrostatic charging is considerably simplified, and the electrical current required can be supplied by small batteries, making the charging apparatus safe and easily portable.

It will be appreciated that the invention is primarily concerned with electrophotoconductive duplicating processes in which a photoconductive surface is electrostatically charged prior to exposure to light. On the other hand, the electrostatic charging apparatus of this invention is importantly useful in transferring powder images from one surface to another and is, indeed, useful wherever electrostatically charged surfaces are desired, as in electrostatic precipitators.

In accordance with the present invention, a pair of oppositely charged electrodes are spaced apart to lie in a single plane and connected to a source of a unidirectional electrical current of high voltage and low amperage. These electrodes are placed in electrical proximity to one another to establish an electrical field between the same. This field is easily observed by suspending nylon flock particles in a paraffinic solvent (Shell Sol 71) and placing the suspension in a glass tube in the field to cause the flock particles to align in a pattern, similar to the way a magnetic field aligns iron filings placed within the magnetic field. At least one of the electrodes is constituted by a friction element of low conductivity which, in use, is rubbed across the surface to be charged. The conductivity of the friction element should be such that when test electrodes are engaged with the element at points 1 inch apart and supplied with 28 /2 volts at 25 microamps, the resistance of the element will be in the range of 200,000 800,000 ohms, most preferably about 400,000 ohms. The surface to be charged is preferably part of an electrically conductive element and the electrical field between the electrodes is augmented by the presence of the electrically conductive element. In this way, the surface to be charged is rubbed by the charged friction element and simultaneously subjected to the electrical field, which is preferably augmented as indicated, and this serves to induce the desired electrostatic charge in the surface which is rubbed.

The friction element is preferably constituted by a layer of asbestos, the chrysotile of the serpentine species of asbestos being outstanding either in felt or woven fabric form. Bamboo can also provide a fiber providing the desired low conductivity, and cotton rendered conductive is also broadly useful, but chrysotile asbestos is uniquely superior. While both electrodes may be surfaced in the same manner, only the electrode which is to impart the desired charge need be surfaced in the manner indicated. The other electrode may be surfaced with any at least partially conductive material.

The oppositely charged electrodes are then moved to draw the charged friction element across the surface to be charged with'the friction element frictionally contacting the surface to be charged. As a result, the surface to be charged is rubbed by the charged friction element and, at the same time, it is passed through the electrical field described hereinbefore. The result is one side charging using high voltage and low amperage and without reliance upon the usual corona or grounded support. A simple flashlight battery is more than ample to provide the needed electrical power. Batteries sized A-AAA are adequate. While some of the fibers in the friction element may emit microcoronas, such microcoronas cannot provide a uniformly charged surface without rubbing as is essential herein. I

In the charging of a negative photoconductive surface, which is the case for zinc oxide-coated paper, the positive electrode is used to complete the electrical field, and the photoconductive surface is rubbed by a negatively charged friction element. The electrically conductive layer on the conventional photoconductor serves to augment the electrical field and this is important to good charging. The opposite electrode arrangement is used to positively charge positive photoconductive surfaces as are common with selenium photoconductors. The bicharge type (organic photoconductors) respond to either technique and can be charged either way. In this way, an electrical current supply to the electrodes of. 100 volts or more, preferably at least 400 volts, and without any grounded support is adequate to obtain a useful level of charge at very low amperage.

From the standpoint of safety, it is preferred to have the power factor low enough to avoid discomfort and sparking. It is particularly preferred to employ at least 400 volts in combination with less than 100 microamperes. Where higher speed is desired, and where contact with human beings is minimized, higher voltages and amperages may be used.

Interestingly, and when the friction element is woven, the weave leaves a pattern of lines on the surface being charged, and this pattern is easily seen in the powder image, establishing the significance of the rubbing action, for where there is no rubbing, there is no adequate electrostatic charging.

With a negatively charged powder image, as for example that noted above, the reverse action across the reverse side of the photoconductive layer causes the powder image to transfer. Thus, and rubbing the back of the imaged sheet with the positive electrode element while the imaged sheet is subjected to the electrical field, causes the powder image to be propelled off the photoconductive surface and onto a paper layer placed adjacent the same.

It will be understood that when the original light exposure produces a mirror image on the photoconductive surface, as is frequently the case, the powder image is a mirror image so that, when thispowder image is transferred as described above, a right-way-round image is produced on the paper layer which receives the same.

The invention will most commonly be used to charge flat surfaces, in which case the single plane of the spaced electrodes is a flat plane. However, and since the surface to be charged may be curved, the single plane referred to may be curved to conform. Also, and while one or both of the electrodes may be stationary, they may also be mounted for movement to enhance the desired frictional engagement with the surface to be charged. Thus, one or both of the electrodes may be agitated or the electrodes may take the form of rollers which may be rolled over the surface to be charged or rotated and moved relative to the surface to be charged while frictionally engaged therewith. Both electrodes are on the same side of the surface to be charged.

The invention will be more fully understood from the accompanying drawings in which:

FIG. 1 is a simplified diagrammatic view showing a charging apparatus in accordance with this invention;

FIG. 2 is a wiring diagram showing an illustrative power supply in detail; and

FIGS. 3 and 4 are a series of diagrammatic views showing the production of a right-way-round copy in accordance with this invention.

Referring first to FIG. 1, the numerals and 11 identify a pair of electrical leads each of which contacts an electrode constituted by a low conductivity friction element identified by the numerals 12 and 13. As previously indicated, chrysotile asbestos constitutes a preferred friction shield. The electrodes 12 and 13 are supplied with a high voltage unidirectional electrical current at low amperage from a battery source of power by means of an electrical circuit not shown in this figure. For illustrative purposes, a low voltage battery (type A-AAA) is typically converted into a supply of high voltage and low amperage, e.g., about 800 volts at 50-60 microamperes. It will be observed that the fric- I I tion elements 12 and 13 (which are the electrodes) lie in a common plane and that these electrodes are in electrical proximity with one another establishing an electrical field therebetween as indicated. It will be un derstood that electrodes 12 and 13 are normally used on top of a conventional photoconductive paper sheet which has an electrically conductive base, and this assists in developing the electrical field which enables good charging.

Referring to FIG. 2, the numeral identifies a 6 volt battery source of direct electrical current which is constituted by four penlight cells. The electrical power is passed through a 50:1 step-up transformer 21. The positive pole of the battery is connected to the transformer through a vibrator 22 which assists in applying peak voltages to the transformer. A switch 23 is supplied so that the charger can be turned on and off as desired. A rectifier 24 and capacitor 25 are placed in parallel across the output of the transformer, and a second rectifier 26 and capacitor 27 are positioned between the output of the transformer '21. A capacitor 28 is also placed in parallel across the vibrator. The output current from the power supply has its positive electrode connected between capacitor 27 and rectifier 26, and the negative electrode is connected between rectifier 24 and capacitor 25.

An auxiliary circuit 30 is provided to test the strength of the batteries and a separate circuit 31 may be provided to enable selecting the voltage of the output.

It will be appreciated that the power supply shown in FIG. 2 is simply illustrative since the power supply system shown is considered to be conventional, and its selection is a matter of convenience.

Referring more particularly to FIGS. 3 and 4, the numeral identifies a photoconductive sheet having a conventional zinc oxide photoconductive layer 41 positioned upon a conductive support 42. Passing the charsure (using a typed original with its indicia facing the sheet 40) and application of powder toner will produce a mirror powder image identified in FIG. 4 by the numeral 45. When the charger 43 is then passed beneath the sheet 40 in the direction shown by arrow 46, the powder image 45 is transferred as indicated by arrows T to a paper sheet 47 to produce a right-way-round powder image thereon which can be fixed as desired. It will be understood that the light exposure, dusting, and fixing steps are wholly conventional and are not shown in FIGS. 3 and 4.

It will be appreciated that the charger 43 in FIG. 3 can be reversed so that the positive electrode is the last electrode to pass over the photoconductive sheet. This is particularly desirable when the photoconductive sheet is constituted by an organophotoconductive resin which is adapted to receive a positive electrostatic charge. This positive electrostatic charge can then be developed using conventional positive toner particles which are repelled by the positive charges which remain on the photoconductive resin. Accordingly, if the light exposure is made to a negative original, then a reversal toning method is provided which is particularly adapted for the reproduction of continuous tone negatives.

It is desired to point out that the electrodes 12 and 13, in the form illustrated, will extend across the entire surface charged so that a single pass of the charger 43 will cause the surface to be charged to be first rubbed by one of the electrodes, and then by the other of the electrodes, the trailing electrode governing the charge which remains on the surface. On the other hand, and once one concludes which charge is to be used, negative or positive, then the exact positioning of the other electrode can be varied considerably, e.g., it need not extend all the way across the surface to be charged and, indeed, it can be positioned to one side if desired. It will also be understood that frictional engagement with the surface to be charged in all of the areas where a charge is desired is essential. This requires that every minute portion of the surface where a charge is desired be actually engaged by the friction electrode. Accordingly, the electrode would normally be provided with a resilient support as by mounting the same on cotton wadding. The rubbing surface is most desirably fibrous so that each individual fiber will have an opportunity to rub. Of course, if the fibers are made into threads which are woven, then, particularly, if the yarns are too large or the weave is too coarse, there will be a tendency for streaking as the electrodes are rubbed across the surface to be charged, but this can be tolerated or overcome by agitating the electrode to insure contact with the entire surface to be charged.

The invention is defined in the claims which follow.

electrodes being constituted by a low conductivity friction element, the conductivity of said element being such that test electrodes placed at points 1 inch apart thereon and supplied with 28 V2 volts at 25 microamps will produce a resistance in the range of 200,000

5. Apparatus as recited in claim 1 in which said electrical current supply to said electrodes is 100 volts or more.

6. Apparatus as recited in claim 5 in which said electrical current supply is at least 400 volts in combination with less than 100 microamperes.

7. A method of electrostatically charging the photoconductive surface of a photoconductive sheet comprising rubbing said surface against an electrode of one charge sign constituted by a low conductivity friction element while passing said surface through an electrical field between said first named electrode and a second electrode of opposite charge sign spaced from said first electrode and positioned on the same side of the surface to be charged, said friction element having a conductivity such that test electrodes placed at points 1 inch apart thereon and supplied with 28 Va volts at 25 microamps will produce a resistance in the range of 200,000 800,000 ohms,.and said electrodes being supplied with a unidirectional electrical current of high voltage and low amperage whereby said surface is electrostatically charged without generating a corona.

8. A method as recited in claim 7 in which said photoconductive sheet is surfaced with zinc oxide and said low conductivity friction electrode is negatively charged.

9. A method as recited in claim 7 in which said friction element is constituted by chrysotile asbestos. 

1. Apparatus for triboelectrostatically charging a surface comprising a pair of oppositely charged electrodes spaced apart to lie in a single plane, means to supply said electrodes with a unidirectional electrical current of high voltage and low amperage, said electrodes being in electrical proximity to one another to establish an electrical field between the same, at least one of said electrodes being constituted by a low conductivity friction element, the conductivity of said element being such that test electrodes placed at points 1 inch apart thereon and supplied with 28 1/2 volts at 25 microamps will produce a resistance in the range of 200,000 - 800,000 ohms, said pair of electrodes mounted in such manner that said pair of electrodes will be positioned on one side only of a surface to be charged.
 2. Apparatus as recited in claim 1 in which said element is constituted by chrysotile asbestos.
 3. Apparatus as recited in claim 2 in which said asbestos is in the form of a felt.
 4. Apparatus as recited in claim 1 in which the power source for supplying said electrical current is a battery.
 5. Apparatus as recited in claim 1 in which said electrical current supply to said electrodes is 100 volts or more.
 6. Apparatus as recited in claim 5 in which said electrical current supply is at least 400 volts in combination with less than 100 microamperes.
 7. A method of electrostatically charging the photoconductive surface of a photoconductive sheet comprising rubbing said surface against an electrode of onE charge sign constituted by a low conductivity friction element while passing said surface through an electrical field between said first named electrode and a second electrode of opposite charge sign spaced from said first electrode and positioned on the same side of the surface to be charged, said friction element having a conductivity such that test electrodes placed at points 1 inch apart thereon and supplied with 28 1/2 volts at 25 microamps will produce a resistance in the range of 200,000 - 800,000 ohms, and said electrodes being supplied with a unidirectional electrical current of high voltage and low amperage whereby said surface is electrostatically charged without generating a corona.
 8. A method as recited in claim 7 in which said photoconductive sheet is surfaced with zinc oxide and said low conductivity friction electrode is negatively charged.
 9. A method as recited in claim 7 in which said friction element is constituted by chrysotile asbestos. 